System and method of AV interval selection in an implantable medical device

ABSTRACT

An implantable medical device provides ventricular pacing capabilities and optimizes AV intervals for multiple purposes. In general, intrinsic conduction is promoted by determining when electromechanical systole (EMS) ends and setting an AV interval accordingly. EMS is determined utilizing various data including QT interval, sensor input, and algorithmic calculations.

BACKGROUND

1. Field of the Invention

The present invention relates to implantable medical devices. Morespecifically, the present invention relates to implantable medicaldevices that are capable of delivering pacing stimuli.

2. Description of the Related Art

At a superficial level, the mechanical aspects of the cardiac cycle ofthe human heart are fundamentally simple. The heart has four chambers.Deoxygenated blood is returned from the body to the right atrium. Theright atrium (RA) fills the right ventricle (RV), which, uponcontraction, pumps blood to the lungs. Oxygenated blood from the lungsfills the left atrium (LA), which in turn fills the left ventricle (LV).The contraction of the left ventricle then delivers oxygenated bloodthroughout the body. Thus, the atrial chambers serve the purpose offilling their respective ventricular chambers.

Similarly, the electrical and timing aspects of the cardiac cycle arealso fundamentally simple, at a superficial level. The sinoatrial node(SA node) is the heart's natural pacemaker and initiates electricaldepolarization of the heart at a predetermined rate, based uponphysiologic need. The SA node is located in the right atrium and uponactivation, the atrial chambers respond to the depolarization byengaging in a muscular contraction. The depolarization wavefronteventually reaches the AV node, where a delay is imparted beforedepolarizing and contracting the ventricles.

The cardiac cycle is often described by atrial (A) events andventricular (V) events. Thus, the activation of the SA node is anintrinsic atrial depolarization. Some time later, the ventriclesdepolarize. There is a delay and the process is repeated. Thus, normaltiming is A-V-A-V, etc. For purposes of understanding the physiology aswell as for programming various pacemakers, this simple understandingprovides several common variables. The rate of the heart is defined by acomplete cycle and may either be an A-A interval or V-V interval (A-Awill be used herein for explanatory purposes). The time between theatrial event and the ventricular event is the AV interval and notsurprisingly, the time between the ventricular event and the subsequentatrial event is the VA interval.

As rate is increased, the A-A interval decreases in duration. The AVnode modifies the delay imparted, thus the AV interval is also reduced.The mechanical actions involved (contraction of a chamber; ejection of afluid) may occur more quickly, but there is a limit or minimal timerequired for efficacious operation.

This highly simplified overview can actually provide for many of the keyprogramming parameters in a given dual chamber pacemaker. Typically, adual chamber pacemaker will have an atrial lead (and electrode)positioned within the right atrium and a ventricular lead (andelectrode) positioned within the right ventricle, generally with theelectrode positioned at the right ventricular apex. Assuming a givenpatient had no intrinsic rhythm and fully relied upon the pacemaker, therate would always be the device's escape interval which defines an A-Ainterval. This interval may be varied by the device based upon sensorinput to provide rate responsive (RR) pacing. An AVI or AV interval isprogrammed and may be varied by the device depending upon rate or otherfactors. AV synchrony is maintained in that a ventricular paced event(VP) will always follow an atrial paced event (AP). A typical DDDpacemaker may operate in this manner.

While the present discussion is overtly superficial both in terms of thecardiac cycle and operation of a pacemaker, several fundamental aspectshave been illustrated that are currently being questioned. The first isthat ventricular pacing in the right ventricular apex may not behemodynamically optimal for all patients. The second is that aprogrammed AV interval that more or less assures ventricular pacing,even to maintain synchrony, is not necessarily optimal in all patients.Finally, the entirety of the above discussion was in terms of RA to RVelectrical timing, which while common parlance tends to ignore a greatmany aspects of the cardiac cycle.

Implanting leads into the right atrium and right ventricle issignificantly easier than implanting leads to pace the left atrium orleft ventricle, as leads on the left side are preferably implantedepicardially or within the veins of the heart proximate, but external tothe relevant left sided chamber. That is, there is a general medicalbias against placing leads or electrodes within the left atrium or leftventricle as this could promote clotting that results in a thrombus.Thus, right side implantation of single or dual chamber pacemakers isthe norm. When dual chamber pacemakers are so implanted, the device istypically programmed to operate in a DDD mode or VDD for a singlechamber, ventricular pacemaker. Such settings restore rhythm, but ensurethat pacing occurs in a high percentage of cardiac cycles.

Ventricular pacing from the right ventricular apex causes thedepolarization wave to travel a rather unnatural path and while it willcause the left ventricle to depolarize, the timing of the left ventriclewith respect to the right ventricle is skewed electrically andmechanically. Recently, there has been recognition that intrinsicconduction is preferable to pacing in most cases. That is, even if theAV delay is longer than “normal,” it is preferable to wait for theintrinsically conducted beat than to pace. This is, of course, at oddswith standard DDD (or similar) modes, which will provide a ventricularpace after a predetermined interval (AVI), which is usually short enoughthat it precludes intrinsic conduction. Certain patients who arepacemaker dependant, e.g., those that have complete heart block, willrequire and benefit from such ventricular pacing. Other patients mayhave pacemakers implanted for other reasons and have intact conductionor may have intermittent block. For those patients, intrinsic conductionis often if not always possible and is typically precluded by standardDDD mode settings.

Again referencing a device having leads in the right atrium and/or rightventricle, the timing relied upon both for programming/discussionpurposes as well as what is sensed by the implanted device is based uponright side electrical timing. The use of right side timing will tend toignore the delays in left sided response that occur naturally and/or asthe result of pacing. In a normal, healthy heart the SA node willdepolarize and generate a wavefront along an atrial conduction pathwaythat eventually reaches the left atrium causing it to depolarize andcontract. The wavefront also reaches the AV node and progresses alongthe Bundle of His. The left sided pathway propagates somewhat fasterthan the right, but because the right ventricle is smaller the wavefrontleads to a generally synchronized mechanical contraction of bothventricles.

When atrial pacing is introduced, the electrode is typically offset fromthe SA node, commonly in the right atrial appendage, and differentconduction (and possibly less efficient) pathways are taken. See U.S.Pat. No. 5,179,949, issued to Chirife, which is herein incorporated byreference in its entirety. The net result is that there is aninteratrial conduction delay (IACD) that is imposed. That is, the leftatrium will depolarize and then contract after a longer interval fromthe pacing pulse than would occur intrinsically, i.e., after the SA nodeinitiates depolarization. Thus, if the remainder of the conductionpathway were intact, this would skew the results for ventricularsensing. That is, an A pace occurs and after some interval, ventriculardepolarization is sensed in the right ventricle by the pacemaker. Thisduration is determined to be the AV interval. However, the left atriumdid not depolarize simultaneously with the A pace, nor within the normalphysiologic window. Thus, the left sided AV (LAV) interval is shorterthan the sensed right sided (RAV) interval. That is, LAV=RAV−IACD.

Another left sided variation to timing occurs when right sidedventricular pacing, particularly at the right ventricular apex isprovided. As indicated, normal ventricular conduction begins at the AVnode and more or less simultaneously propagates along a left and rightside of the Bundle of His and spreads around each ventricle. With rightsided pacing, the normal conduction pathway is not necessarily activatedand instead propagation from cell to cell may occur at a slower rate. Inaddition, the wavefront propagates from the apex retrograde along theBundle of His, then down the left side pathway eventually depolarizingthe left ventricle. The delay imparted from the ventricular pace tocontraction of the left ventricle is referred to herein as theinterventricular conduction delay (IVCD).

Yet another offset is the difference between an event, e.g., an atrialdepolarization, and the time at which that event is sensed by thepacemaker. This delay is referred to as the P wave sense offset (PSO).

These various delays are biased towards right side events. That is,failing to account for such delays may have the most consequence on leftside activity, which is generally more important to hemodynamicperformance.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is schematic, partially sectional view of a human heart.

FIGS. 2A-2E illustrate the mechanical contractions of a heartprogressing through a cardiac cycle.

FIG. 3 is illustrates a correspondence between depolarization eventsoccurring throughout a heart and how those events generate a compositeEKG tracing.

FIG. 4 is a graph illustrating correlations between a surface EKG,pressure data, volume data and heart sounds.

FIG. 5 is a series of stylized EKG tracings.

FIG. 6 is a block diagram of a first embodiment of the presentinvention.

FIG. 7 is a block diagram of a second embodiment of the presentinvention.

FIG. 8 is a block diagram of a third embodiment of the presentinvention.

FIG. 9 is a graph illustrating device rate limits.

FIGS. 10-13 illustrate stylized pressure waveforms and corresponding EKGdata.

FIGS. 14-16 are flowcharts illustrating processes utilizing embodimentsconsistent with the teachings of the present invention.

DETAILED DESCRIPTION

FIG. 1 is schematic, sectional diagram illustrating the anatomy of ahuman heart 10. Blood returning from the venous system flows into theright atrium 20 from the superior (SVC) 50 and inferior vena cava. In anormal, healthy heart 10, the sinoatrial (SA) node 22 produces an actionpotential that is responsible for the automaticity of the cardiacconduction process. A depolarization wavefront is generated and progressthrough the right atrium (RA) 20 along the atrial conduction pathway 26to the atrioventricular (AV) node 24. At the same time, thedepolarization wavefront propagates from the SA node 22 into the leftatrium (LA) 40 along the RA to LA conduction pathway 30. Thedepolarization wavefront, generally referred to as the P wave triggers asubsequent muscular contraction as it propagates. That is, theelectrically detected wavefront, i.e., an EKG, is not identicallysynchronized with the mechanical contraction.

As blood is filling the RA 20, the LA 40 is similarly being filled viathe pulmonary veins. During this time, the right ventricle (RV) 46 andthe left ventricle (LV) 54 are being passively filled with blood flowingthrough the tricuspid valve 52 and the mitral valve 56, respectively.When the atrial chambers mechanically contract, they force an additionalvolume of blood into the ventricles, causing the ventricles to stretchsomewhat. This is referred to as atrial kick and improves overallcardiac output.

When the atrial electrical wavefront reaches the AV node 24, a delay isimparted. This delay provides time for the atrial chambers to contractand fully fill the ventricles, prior to ventricular contraction. Afterthis delay (the PR interval), the depolarization wavefront progressesdownward through the ventricular septal wall along the Bundle of His 32and splits into the left bundle branch (LBB) 35 and right bundle branch(RBB) 36. The bundle branches diverge proximate the apex 42 of the heart10 and propagate along the Purkinje fibers 41 surrounding theventricles. Due to the atrial kick, the ventricles are expanded orstretched somewhat. As the muscular contraction occurs in response todepolarization, the fluid pressure within the ventricles increases andcauses the tricuspid valve 52 and mitral valve 56 to close. Thecontinued contraction ejects a large percentage of the fluid from theventricles in a coordinated action. After cardiac cells depolarize, theyare refractory for a period of time. The contracted chambers relax andthe process repeats.

FIG. 2 illustrates some of the mechanical and fluid flow characteristicsgenerated during the cardiac cycle. In FIG. 2A, the heart 10 is enteringsystole and is in the period of isovolumic contraction. The AV valves100 (mitral and tricuspid) and semilunar valves 110 (aortic andpulmonic) are closed at this point. In FIG. 2B, systole continues withthe period of ejection where the outflow of blood opens the semilunarvalves 110. FIG. 2C illustrates the onset of diastole and the period ofisovolumic relaxation, where all valves (AV and semilunar) are againclosed. As diastole continues (FIG. 2D), passive ventricular fillingoccurs as the pressure within the atrial chambers causes the AV valves100 to open. During active filling (FIG. 2E), the atrial chamberscontract and further fill the ventricles. The process then returns toFIG. 2A and repeats.

In describing the cardiac cycle, both electrical and mechanicalreference points have been described. Typically, the cardiac cycle isrepresented electrically and most accurately in that context by asurface EKG. Electrodes on the surface of the skin detect, over multiplevectors, the electrical signals generated as depolarization occurs. Acomplete cardiac cycle includes a P wave indicative of atrialdepolarization, a QRS complex indicative of ventricular depolarization,and a T wave indicative of ventricular repolarization. FIG. 3illustrates the potential along the conduction pathways and how eachportion contributes to the overall represented cycle. Of note, theelectrical representation does not indicate any mechanical effect. Thatis, there is a difference between the onset of electrical activity andmechanical contraction. Likewise, the electrical signals do notrepresent either the strength of efficacy of a given contraction.Finally, the representative EKG signal is a summation of electricalactivity and does not readily allow for differentiation of certaincomponents.

FIG. 4 is a timing diagram that illustrates the correlation in time ofan idealized EKG tracing A, pressure graph B, ventricular volume graphC, and heart sound graph D. The pressure graph B includes left atrialpressure P1, left ventricular pressure P2 and aortic pressure P3. Attime T1, the surface EKG indicates initiation of the P wave (atrialdepolarization); however, there is a delay until LA contraction occursand begins to increase atrial pressure P1 at time T2. Prior to thispoint, the LV has been passively filled and with the LA contractionbeginning at time T2, a maximum or end diastolic volume is reached attime T5.

Ventricular depolarization begins with the initiation of the QRS complexat time T3. Isovolumic contraction of the LV begins after a delay attime T5 and the pressure within the LV causes the mitral valve to close.Pressure P2 within the LV increases but the semilunar valves remainclosed until this pressure exceeds that within the aorta P3 at time T6.When the LV pressure P2 exceeds aortic pressure P3, the semilunar valvesopen at time T6. LV pressure P2 continues to increase untilapproximately time T7 and then begins to drop. The period of ejectionoccurs between time T6 and T8 and LV volume (graph C) falls as blood isejected from the LV into the arterial system. The T wave, orrepolarization, begins during the period of ejection, as illustrated.

When the falling LV pressure P2 is exceeded by aortic pressure P3, thesemilunar valves close at time T8. The time period referred to asisovolumic relaxation occurs between times T8 ad T9. During this time,LV pressure P2 is falling rapidly, but is still in excess of LApressure; thus keeping the mitral valve closed. At time T9, the LVpressure P2 falls below the LA pressure P1 and the mitral valve opens.This begins the period of passive ventricular filling that occurs upuntil time T11.

A second cardiac cycle is illustrated with the initiation of the P waveat time T10, initiation of active LV filling at T11, start of the QRScomplex at time T13, R wave at time T14 and isovolumic contraction fromT15-T16. The T wave begins at time T17 and isovolumic relaxation beginsat time T18. Thus, one complete cardiac cycle may be defined from afirst P wave at time T1 to a second P wave at time T10. Similarly, thecycle may be defined by a first R wave at time T4 to a second R wave attime T14. The surface EKG illustrates the intition of a P wave at timeT1. It should be appreciated that for a variety of reasons, an implanteddevice might not sense this same P wave at time T1. Rather, theimplanted device will sense the P wave at time T1+PSO, where PSO is aP-wave sense offset. Depending upon lead placement and location, sensedactivation is not necessarily the earliest actual activation. While thisdoes not change the times at which the other events occur, it doeschange how the electrical representation or at least a portion thereofwould be shifted with respect to the actual occurrence of these otherevents.

Cycle length (CL), as used herein is simply the length of a given cycle(e.g., T1 to T10) and may vary on a beat to beat basis due tophysiologic demand for an intrinsic rate or controlled by a pacemakerbased upon programmed parameters and various sensory input. As cyclelength decreases, the AV interval (T1 to T3) is able to slightlydecrease. Similarly, ventricular diastole may be shortened.

Such variation based upon dynamic cycle lengths is normal within certainparameters. The present invention relates to promoting intrinsicconduction without permitting these variations from exceeding thosenormal parameters. As illustrated in FIG. 4, ventricular systole occursfrom time T5 with isovolumic contraction and ends at time T8, whenisovolumic relaxation begins. This is the time period during which theleft ventricle is actually contracting. As used herein, the termelectromechanical systole (EMS) begins with either the pacing pulse or Rwave depending upon the embodiment, and ends when passive leftventricular filling begins at time T9. In other words, EMS includes atime period initiated by the pacing pulse (or sensed R wave), mechanicalsystole, and the period of isovolumic relaxation. In some embodiments,EMS ends at time T8 or at a time between T8 and T9, as described herein.

FIG. 5 illustrates three timing diagrams, schematically illustratingselected events at various times for different heart rates. In eachscenario, the AV interval is constant. In scenario A, the heart rate isassumed to be 70 beats per minute (BPM). At time T1, an atrial pace isdelivered with an R wave occuring at time T2. The EMS is illustratedthrough time T3 and a second atrial pace is delivered at time T4. Thetime difference between T3 and T4 is sufficient for ventricular passivefilling (see FIG. 4). In scenario B, the heart rate is 90 BPM; thus, theA-A interval is shorter than in scenario A. Here, the second A pace isdelivered at time T4, very close in time to the end of EMS at time T3.This will overlap the period of passive ventricular filling with activefilling due to left atrial contraction. Finally, in scenario C, theheart rate is at 110 bpm. As such, the atrial pace is delivered (T3)during the EMS which ends at T4, causing truncation of active atrialflow.

There are two undesired results that may occur as an atrial eventencroaches the EMS. The first is that the period for active fillingoverlaps with passive filling rendering left atrial contraction lessefficacious. The second is that atrial contraction occurs (in full or inpart) during the EMS. During this time, the ventricular pressure ishigher than the atrial pressure. Thus, even with atrial contraction,insufficient pressure is generated to open the AV valves; thus, fluidflow is precluded to the ventricles but does occur retrograde into thepulmonary veins and pulmonary capillary vessels. Mean pulmonary venouspressure will increase and may result in fluid passing from the veinsinto the pulmonary tissue, impeding normal gas exchange. Symptomssimilar to that of heart failure may occur or preexisting pulmonaryedema may worsen. The condition may be referred to as pacemaker syndromeor pseudo-pacemaker syndrome if no pacemaker is implanted.

As illustrated, in FIG. 5, this occurs at a higher atrial rate with aconstant AV. More accurately, the AV interval (or PR interval) is toolong for scenario C; thus leading to the second A pace occuring duringthe EMS. It should be appreciated, that abnormally long AV intervals mayresult in the same or similar symptoms even in the absence of atrialpacing.

This situation is commonly averted through the use of a pacemakeroperating in a traditional DDD mode. A simple solution is to utilize thestandard DDD pacing mode with relatively short AV interval to avoidencroachment; however, as previously mentioned, intrinsic conduction ishighly preferable to ventricular pacing when possible. The scenariosillustrated in FIG. 5 indicate atrial pacing and intrinsic R waves. Evenassuming that the AV interval is longer than “normal” in all threescenarios, intrinsic R waves occur and at lower heart rates, result inhemodynamically effective cardiac cycles. If a DDD mode were utilized,then a ventricular pace would likely have occurred prior to theintrinsic R wave, precluding the benefits of intrinsic conduction.

On the other end of the spectrum is the use of an AAI pacemaker. In sucha case, there is no ventricular sensing or pacing capability; thus,intrinsic conduction is not only fully promoted it is entirely reliedupon. In the absence of long AV interval and while conduction is intact,this is a beneficial selection. Of course, with prolonged AV intervals,the encroachment illustrated in FIG. 5 could occur and if heart blockoccurs, there is no mechanism to provide ventricular pacing.

Thus, the present invention provides a pacing mode that promotesintrinsic conduction to a high degree while providing properly timedventricular pacing, when required to generally prevent cardiac cyclesdevoid of ventricular activity. The present invention determines theEMS, which varies based on rate, and calculates a maximum AV interval(AV_(max)). The AV_(max) results in an AV interval for any given cyclethat allows approximately the longest delay possible to promoteintrinsic conduction while still being able to deliver a ventricularpacing pulse, leading to left ventricular contraction that is notencroached upon by a subsequent left atrial contraction.

With reference to FIG. 4, the EMS is a combination of electrical andmechanical events that are not readily identifiable by implanteddevices. It should be appreciated that the idealized representationpresented in FIG. 4 is not simply reproducible by an implantable device.For example, the EKG illustrated is a surface EKG. Implantable devicesgenerate an electrogram (EGM), which identifies certain electricalactivity along selected vectors and in specific sites within the heart.In addition, the PSO will shift the correlation of detected electricalevents. Left atrial and left ventricular pressures are not readilymeasured in general, and certainly not available to a typical dualchamber (right sided) pacemaker. Likewise ventricular volume data is notavailable to a typical implanted device. To the extent measurements aremade for a given patient they are done via imaging techniques such as anechogram in a controlled environment resulting in a limited data set.

FIG. 6 is a schematic diagram of an implantable pacemaker 300. It shouldbe appreciated that reference to pacemakers would also includeimplantable cardioverter-defibrillators (ICDs) or similar devices havingpacing capabilities. The pacemaker 300 includes various components knownin the art and not illustrated herein for clarity, such as amicroprocessor, memory, batteries, capacitors, sensors, telemetrycomponents, and the like. An atrial lead 310 and ventricular lead 315are schematically illustrated as being electrically coupled with a pulsegenerator 312.

The present embodiment operates in a novel mode having somecharacteristics similar to DDD/R. VDD/R is another similar mode and itshould be appreciated the reference to DDD/R herein would include VDD/Rembodiments as appropriate, without further mention. In traditionalversions of these modes, an AV interval is set to represent nominal AVdelay and provide a long VA (relatively speaking) to avoid the issuesdescribed above. As noted, this typically will preclude intrinsicconduction. With the present mode, referred to herein as EMS basedDDD/R, the above described EMS is measured, calculated or determined anda maximum AV interval is provided. In this manner, intrinsic conductionis fully promoted for each cycle while avoiding encroachment issues.Furthermore, as left sided effects are more hemodynamically importantthan right sided effects, the EMS based DDD/R will provide for delays oroffsets in left sided timing generated by right sided sensing and/orpacing.

With reference to FIGS. 4 and 6, the timing of the EMS, as describedabove is approximated by the QT interval. Thus, the pacemaker 300determines the QT interval by sensing electrical activity through theventricular lead 315. The sensed data is provided to a QRS amplifier 320and the output of the amplifier 320 is provided to a signal processor325. Measurement of the QT interval in and of itself is known andvarious methods are available. Sensing of the QT interval may involvesensing the intition of the T-wave, initiation of the T-wave with anappropriate offset (similar to the PSO), sensing of a peak amplitude ofthe T-wave to define a midpoint or median, sensing a point within thesecond slope of the T-wave (negative normally, positive with invertedT-waves), sensing an endpoint of the T-wave, or calculating/extractingan endpoint of the T-wave using one or more of the above methods. Inanother embodiment, the patient's QT intervals are clinically measuredand an appropriate offset is provided from a point sensed by the deviceto the determined end of the T-wave. Thus, in subsequent sensing the Twave is sensed and the end of the T wave is calculated.

A signal is provided to a first clock 330, which is responsible forproviding a prevailing cycle length CL. This data may be obtained fromthe ventricular lead 315 by sensing the duration of R-R intervals.Alternatively, other inputs may be provided from an atrial input for A-Aintervals or a data location within the pacemaker 300 that indicates thecurrent escape interval. In any event, the first clock 330 represents ameans for obtaining a value for a cycle length CL. Ac

The signal processor 325 provides data to QT detection module 340, whichincludes or is in communication with a second clock 345. The QTdetection module 340 and clock 345 are responsible for providing a valuefor the measured or sensed QT interval. The QT interval may begin with asensed ventricular event or a delivered ventricular pacing pulse. Thevalues for the cycle length CL and QT interval are provided to acalculation module 350 that determines a maximum AV interval, defined asthe cycle length minus the QT interval. This AV_(max) value is providedto the pulse generator 312, which sets an AVI (AV interval) equal to theAV_(max) value. Thus, in the next cycle an atrial event (paced orintrinsic) initiates the AVI. This AVI will at least approximate thelongest permissible interval to wait for intrinsic conduction withoutnegatively interrupting left atrial transport. It should be appreciatedthat the duration of the EMS and QT interval are based upon rate,patient specific parameters, inotropic state and other variables. Thus,AV_(max) is calculated on a beat to beat basis.

The AV_(max) is the interval that begins with an atrial event, likely anatrial pacing pulse, and is either terminated by a sensed ventricularevent or upon expiration of the interval, a ventricular pacing pulse isdelivered. As indicated, this most commonly occurs with a ventricularpacing lead placed within right ventricle. Similarly, the atrial pacingpulse is most commonly delivered via a lead placed in the right atrium.As such, the calculation provided in the calculation module will beAV_(max)=CL−QT+IAD where IAD is the interatrial delay. There are othervariables included in the calculation, as discussed below.

With reference to FIGS. 4 and 7, a second embodiment is shown. Apacemaker 400 includes an atrial lead 402 and a ventricular lead 404coupled with a pulse generator 406. A sensor 408 is illustrated with anappropriate amplifier 410 and signal processor 412. The output from thesignal processor 412 is used in a determination of the end of EMS. Theclosing of the aortic valve is a physiological marker indicative of theend of EMS. Thus, sensor 408 in one embodiment is a microphone,accelerometer, vibration sensor or similar sensor that detects heartsounds. The closure of the aortic valve causes the second heart sound(S2). Thus, the output of the signal processor 412 is provided to aheart sound detector 414 in communication with clock 416 that triggersthe end of the EMS. Again, AV_(max)=CL−EMS+IAD and is calculated inmodule 418. A second clock 420 is coupled with the pulse generator 406and module 418. The cycle length is provided to the module either as theescape interval of the pacemaker 400 or sensed rate. The sensed R waveor ventricular pace initiates the clock 416, which terminates upon anindication of the second heart sound. It should be appreciated that thefirst heart sound (S1) is caused primarily by the closure of the AVvalves, proximate in time to the QRS complex, the second heart sound(S2) is readily distinguishable as there is a delay of 100-300 msbetween the first and second heart sounds.

FIG. 7 illustrates an embodiment that uses one or more sensors to detectheart sounds. The present embodiment alternatively or additionallyincludes the use of other sensors to identify other physiologicalmarkers that indicate the end of EMS. For example, a pressure sensorwithin the left ventricle or in another anatomical location (e.g., RV)with an appropriate offset, can be used to detect peak negative dP/dt ofventricular pressure. While not exactly correlating to the end of EMS,it could be used as an approximation. Similarly, measuring aorticpressure P3 provides a readily identifiable marker of valve closure,which does correspond to the end of the EMS. Again, direct measurementof these pressure values on the left side is difficult, but if availablecould be used in the calculation.

Another alternative sensor is an impendence sensor. By generating asub-threshold electrical signal via one or more pacing leads and/or thecan electrode, intracardiac impedance can be measured. U.S. Pat. Nos.4,719,921 and 5,154,171 relate to impedance measurement and are hereinincorporated by reference in their entireties. The end of systole ismarked by minimum ventricular volume, which causes the maximumimpedance. Thus, impedance measurements may be used to determine EMS. Ifpressure sensing and/or impedance sensing is utilized, such a sensorwould be indicated by sensor 408 in FIG. 7; thus, sensor 408 could be amicrophone, vibration sensor, accelerometer, impedance sensor, flowsensor, and/or pressure sensor. While a microphone is illustrated, itshould be appreciated that alternative circuitry would be provided toobtain useable signals for other sensor types.

With reference to FIGS. 4 and 8, another embodiment is illustrated. Aspreviously indicated, the EMS parameters are not readily available tocurrent pacemakers or other implantable medical devices. The previousembodiments described herein have provided mechanisms to determine theEMS and then utilize that value in various pacing parameters. Thepresent embodiment provides a mechanism to estimate the EMS on a dynamicand on-going basis without a direct measurement or indicator of theactual EMS. As indicated, the EMS will vary based upon patient specificparameters, interventricular delays, inotropic state, and rate. Thepacemaker 500 of FIG. 8 includes a plurality of registers 530 simplyrepresenting values that may be stored in memory. The pacemaker 500 isprogrammed with a medical device programmer 510 via a telemetry systemas is known in the art. In summary, at the time of implant or shortlythereafter, clinical observations and measurements are made to providecertain baseline data that is provided to the pacemaker 500. Thepacemaker 500 uses this baseline data in combination with sensed orknown dynamic data (e.g., cycle length) to determine an appropriate EMS.

A baseline parameter of a resting EMS (EMSr) is made and stored inregister A. The resting EMS can be determined in a clinical settingusing, for example, Doppler imaging to observe aortic flow, M-modeechocardiography to identify the closure of the aortic valve,catheterization to measure pressure values, QT interval measurement, orphonocardiography to identify the second heart sound. Of course, in aclinical setting there are a number of mechanisms available to make thesame or similar observations. By using an EKG or data from the implanteddevice, the initiation of the EMSr is determined by a sensed R wave, QRScomplex, or pacing pulse with the end of the EMSr determined via one ofthe above measurements. In cases where such measurements cannot be made,nominal values may be utilized based upon generalized clinicalobservations. A default value for EMSr of about 450 ms may be used forpatients having a narrow QRS complex, while a value of 530 ms may beused for patients having left bundle branch block or who require rightventricular pacing.

As indicated, this is a baseline or resting EMSr; thus, the measurementsare preferably made while the patient's rate is at a resting rate, whichmay be the lower rate of the device. Whatever rate is chosen (or occurs)for the resting rate observation, that rate is recorded in register B asthe resting cycle length or CLr. Register C is the prevailing or currentcycle length CLp. Register D is the interventricular delay IVD that canbe measured clinically, Register E is the interatrial conduction delayIACD, having an intrinsic value and an atrial paced value and Register Fis the P-wave sense offset.

In use, the prevailing EMS (EMSp) is utilized. The EMSp is dynamic andwill change on a beat to beat basis or may be varied at differentincrements. For example, rate ranges may be utilized where a determinedEMSp is utilized so long as the rate remains within that range. Ingeneral, the EMS is proportional to cycle length in that a givenpatient's EMS will decrease as cycle length decreases with about a 1:5correlation. That is, for every 5 ms that the CL decreases, the EMS willdecrease about 1 ms. In one embodiment, a specific correlation of 0.22is utilized, based upon clinical observation.

Thus, calculation module 540 utilizes the EMSp to determine AV_(max) fora given patient at a particular rate. As indicated,AV _(max) =CLp−EMSp−IVD+IAD+PSOEMSp=(EMSr−((CLr−CLp)×0.22))Using these equations determines the maximum permissible AV interval fora given patient at a given rate that will not interfere with the atrialtransport mechanism. The term “maximum” as used herein is meant toindicate approximate or relative values, based upon the efficiencies andcapabilities of a given embodiment and is not meant to reference anabsolute theoretical or actual maximum specific value.

FIG. 9 is a graph that illustrates certain programmable values for agiven pacemaker. As is known, a pacemaker will have a lower rate and anupper rate. The lower rate is the longest permissible cardiac cyclelength (e.g., A-A interval) that the pacemaker will permit. Conversely,the upper rate is the fastest rate at which the pacemaker will pace. Thevalues set for these parameters depend upon patient specific parameters.For example, a young, otherwise healthy patient may be expected toexercise vigorously and attain a relatively high heart rate, as opposedto an older, sedentary patient who has little physical activity. Thelower rate is the rate applied during period of non-exertion orrelaxation. Some pacemakers will have an even lower rate setting (notshown) that is used for when the patient is asleep.

The present invention introduces the programmable activities of dailyliving (ADL) rate. The area between the Lower Rate and the ADL Rateinclude rates appropriate for complete rest or inactivity throughnormally mild physical exertion. For example, a Lower Heart Rate may be60 bpm. Walking, climbing stairs, household chores and similaractivities will typically raise the heart rate, but typically not to alevel consistent with prolonged strenuous work or exercise. Theprogrammed value for the ADL will, of course, be patient specific, butas an example a Lower Rate might be 60 bpm, an Upper Rate might be 170bpm and an ADL Rate may be 90 bpm. In this example, it is presumed thatthe patient will perform normal daily activities with a heart rate at orbelow 90 bpm.

As previously discussed, there is a recognition that intrinsicallyconducted ventricular beats are generally preferable to providing rightsided ventricular pacing, even if the intrinsic AV is somewhatprolonged. The above-described embodiments determine a maximum AVinterval that can be permitted in any given cardiac cycle. That is, thelongest period of time that the device can wait for intrinsic conductionbefore delivering a ventricular pacing pulse. If a ventricular pacingpulse is delivered after this time or an intrinsic event occurs afterthis time, the ensuing atrial pace (or atrial event) may occur duringEMS and atrial transport block may occur.

This dynamic imposes a binary decision on a per cycle basis. That is,each cardiac cycle is divided into two windows; the first is theAV_(max), during which ventricular activity is permitted and the EMSwindow (as opposed to the actual EMS which implies ventriculardepolarization/contraction). The present invention recognizes fourcategories that are referred to with respect to the atrial event for agiven cycle. The four categories are defined as contributory P-waves,deleterious P-waves, wasted P-waves and sacrificed P-waves.

FIG. 10 is graph illustrating a stylized EKG tracing 615 synchronizedwith stylized left atrial pressure LAP and left ventricular pressure LVPtracings. FIG. 10 illustrates a contributory P-wave. At time T1, the EKGindicates atrial depolarization. At time T2, left atrial pressure, asindicted by the LAP tracing, begins to increase. Thus, the intervalbetween T1 and T2 is the IAEMD 600. At time T3, the QRS complex beingsand left ventricular pressure begins to rise at time T4. The intervalbetween T2 and T4 is the left sided AV interval (initial left atrialpressure increase to initial left ventricular pressure increase). Asindicated in FIG. 10 and in the stylized tracings of FIG. 4, this isideal timing. After a sufficient period of time for passive filling, theleft atrium is able to fully contract and just after this contraction,the left ventricle begins to contract. A contributory P-wave is alwaysdesirable.

FIG. 11 represents a deleterious P-wave. The same intervals arerepresented by the same reference numerals; however, the timing isdifferent. At time T1 (approximately), the QRS complex begins. At timeT2, left ventricular pressure begins to rise. During left ventricularsystole, the P wave is sensed at time T3 and left ventricular pressurebegins to rise at time T4. The left ventricle enters diastole atapproximately time T5. Thus, almost the entirety of the mechanicalatrial contraction occurred during ventricular systole. Thus, the leftatrium was not able to deliver blood into the left ventricle during thecontraction, resulting in fluid backup into the pulmonary system. Thisleads to increased stress on the atrium and increased pressure.Deleterious P-waves can vary from the illustrated example, but ingeneral left atrial contraction occurs during or just after ventricularactivation such that the atrial contraction contributes little ornothing to ventricular filling. Deleterious P-waves are undesirable andthe above embodiments are provided to preclude or greatly reduce theiroccurrence.

FIG. 12 illustrates what is referred to as a wasted P-wave. Here, theleft atrial contraction occurs too early in ventricular diastole toprovide meaningful atrial transport and in general, does not contributeto increased cardiac output. At time T1, left ventricular pressurebegins to rise. At time T2, the P wave occurs and left atrialcontraction begins at time T3. By time T3, the left ventricle hasentered diastole, thus the left atrium is not contracting against aclosed mitral valve and as such, does not generate pulmonary back flow.However, there has been little or as in this example, no time providedfor passive filling and no “atrial kick” is provided, thus the P wave isconsidered wasted. Intrinsic conduction is however, facilitated.

FIG. 13 illustrates a sacrificed P-wave. At time T1, an A pace isdelivered and at time T2, left atrial pressure begins to rise. In thisexample, the left ventricle entered diastole some time prior to T1, sosome passive filling has occurred. The sensed QRS complex (intrinsic)occurs at time T3 and left ventricular pressure begins to rise at timeT4. Here, the atrial contraction is still too early in diastole toprovide meaningful atrial transport; however, intrinsic conduction isfacilitated.

In another aspect of the present invention, the above describedembodiments are further modified to promote contributory P-waves, avoiddeleterious P waves, and to permit sacrificed P-waves when the patient'sheart rate is at or below the ADL rate. In this manner, intrinsicconduction is always promoted to the AV_(max) when at or below the ADLRate, but above the ADL Rate, efficacious or contributory P-waves arepromoted. Even while above the ADL Rate, the AV interval may be longerthan that of standard DDD/R or VDD/R pacing to promote intrinsicconduction.

FIG. 14 is a flowchart illustrating a process for using an algorithmicbased, prevailing EMS to promote intrinsic conduction. The processbegins by initializing (700) the sensed AV (SAV) interval and paced AV(PAV) interval values for the implantable medical device (IMD). Itshould be appreciated that the implanted device may make some or all ofthe calculations or an external medical device programmer may make someor all of the calculations and provide data to the IMD. Theinitialization subroutine (800) begins by pacing and/or sensing (810)events in the atrium. During this time, the pacemaker will operate toprovide standard therapy, unless modified to obtain specific data.During this initialization process, the device calculates (812) valuesfor SAV_(norm) and PAV_(norm), which indicate the values that will fullypromote contributory P-waves. The formulas for the various calculationsare indicated in box (702). Specifically, SAV_(norm)=85ms+IAEMDs−PSO−IVD, where IAEMDs is the interatrial electromechanicaldelay for physiologic or intrinsic atrial depolarizations, PSO is theP-wave sense offset and IVD is the interventricular conduction delay(also referred to as the IVCD). When an intrinsic ventricular event withnarrow QRS is sensed, as opposed to pacing the ventricle, the IVD equalszero (0). PAV_(norm)=85 ms+IAEMDp−IVD, where IEAMDp is the interatrialelectromechanical delay when atrial pacing occurs. It should beappreciated that these variables may be measured for a specific patientor nominal values may be used if measurements are unavailable. The 85 msis the mean normal value of the mechanical left heart AV intervalresulting from clinical studies. It is merely an example and not meantto be limiting, since the normal range extends from 60 to 110 ms in oneembodiment 65-105 ms in another.

The patient's resting EMS (EMSr) is measured (814) in a mannerpreviously discussed. This value is then used to calculate (816) theSAV_(max) and PAV_(max). The SAV_(max)=CL−EMSs+IAEMDs−PSO. The EMSsmeans the EMS used in a R-sensing situation and equalsEMSr−((CLr−CL)×0.22) where CLr is the cycle length occurring when theEMSr was determined and CL is the current or prevailing cycle length.The PAV_(max)=CL−EMSp+IAEMDp, where EMSp=EMSs+IVD. The difference isthat when ventricular pacing is provided (EMSp), the interventricularconduction delay (IVD) needs to be considered. The initializationprocess ends by setting the SAV and PAV of the device to the determinedSAV_(max) and PAV_(max).

Now the process is described with the device having been initialized. Anatrial event will occur to begin a cardiac cycle and is either a sensedevent or an atrial pace. The nature of the event determines whether thePAV or SAV interval is used in a given cardiac cycle. During the SAV orPAV, the device senses the ventricles to determine (710) if intrinsicconduction occurs. Assuming the SAV or PAV expires without a ventricularsense, then a ventricular pacing pulse is delivered (720). The devicethen senses for intrinsic atrial depolarization or provides an atrialpace at the end of the escape interval (722). Though indicated as asubsequent step(s), it should be appreciated that the followingcalculations may be performed on a beat to beat basis and do notnecessarily occur after the atrial event (722). In steps (724) and (726)all of the variables determined during initialization (800) arerecalculated with the exception of the EMSr, which is fixed. The EMSsand EMSp values used in the SAV_(max) and PAV_(max) are recalculated.

The device determines (728) if the current heart rate is greater thanthe programmed ADL rate. If the current heart rate is at or below theADL rate, then the device continues to employ SAV_(max) and PAV_(max)values. As indicated, these provide the longest period during any givencardiac cycle for intrinsic conduction to occur without interfering withatrial transport, though sacrificial P-waves could occur. The devicethen progresses (752) into the next cardiac cycle and the process isrepeated from step (710).

Returning to step (728), if the current heart rate exceeds the ADL rate,the device determines (730) whether a search for intrinsic conductionshould occur. This search will be described shortly; however, assumingit is not an appropriate time for a search or this is the first intervalwith the heart rate exceeding the ADL rate, the device sets (732) thevalues for SAV and PAV to SAV_(norm) and PAV_(norm) respectively.

As previously indicated, these are shorter AV intervals (as compared tomax values), so intrinsic conduction is given less chance while adelivered ventricular pace is more optimally timed. In other words,following the flow of this logic diagram, to get to this point thiscardiac cycle has received a ventricular pace. The previous cycles mayhave likewise been paced; thus, there is at least an increasedlikelihood that the next cardiac cycle will have a ventricular pace. Assuch, if there is to be a ventricular pace and the heart rate is abovethe ADL, then setting the SAV and PAV to SAV_(norm) and PAV_(norm)respectively causes the delivered pacing pulse to occur after anormalized AV interval, rather than a maximized AV interval. It shouldbe appreciated, that this feature can be disabled through the medicaldevice programmer, in some embodiments. Thus, the physician may chooseto utilize SAV_(max) and PAV_(max) regardless of heart rate.

If at step (728), the current heart rate is at or below the ADL, thenthe SAV and PAV are set to SAV_(max) and PAV_(max) as the promotion ofintrinsic conduction takes precedence. The next cardiac cycle begins(752) and the process repeats.

When the ADL is greater than the current heart rated at step (728), thenext step is to determine if a search should be performed (730). Asindicated, if this is negative, then the values are set to the “norm”values in step (732). If the device determines that it is time toperform a search, then the SAV and PAV are set to SAV_(max) andPAV_(max) (739) and the next cardiac cycle (752) proceeds with theselonger AV intervals. This provides an opportunity to return to intrinsicconduction, if for example, transient heart block had occurred but wasnow terminated.

The search (730) is conducted at periodic intervals. In one embodiment,a search is performed in the cardiac cycle immediately subsequent to thefirst cycle that has a ventricular pace. Then, assuming ventricularpacing is occurring with each iteration of step (720), the searches areperformed after progressively longer intervals. For example, after 30seconds, after one minute, after 2 minutes, then 4, 8, 12, 24 minutes,etc. Unless the ADL is set inappropriately for a given patient, they areunlikely to sustain heart rates above the ADL for excessively longperiods of time. Thus, if the heart rate is above the ADL for what thephysician may choose as an excessive period of time, this data may bereported as a potential error or indicator of a physiologic concern.Perhaps a young, very active person may have a physically demanding jobwhere their heart is elevated for longer periods of time. Such data maysimply be noted or the ADL may be raised for this patient. Conversely,if a patient who has very limited physical capabilities is above the ADLfor a prolonged period of time, this may indicate a concern. It shouldbe appreciated that the present device and the present algorithms arenot meant to address tachyarrhythmias. It should be understood, however,that the device may very well have algorithms to address and/or providetherapy for tachyarrhythmias and this is separate from the evaluationoccurring at step (728). That is, if the device detects atachyarrhythmia, the therapy may involve departing from the presentalgorithm and providing anti-tachy pacing, defibrillation or othertherapies.

If a ventricular event is sensed during the AV for that cycle (710), theprocess proceeds to step (736). Steps (736), (738) and (740) are thesame as steps (722), (724), and (726) respectively. Subsequent to step(740), the device sets the SAV and PAV to SAV_(max) and PAV_(max),respectively. Absent ventricular pacing, this is not a change other thanthe intervals for those variables are recalculated with each cycle(740). Alternatively, if during a previous cycle ventricular pacing hadoccured and the SAV and PAV were set to SAV_(norm) and PAV_(norm) (732),the step (734) represents a change to SAV_(max) and PAV_(max).

To summarize, once the device sets the SAV and PAV to SAV_(norm) andPAV_(norm), there are three ways in which the algorithm will return toSAV_(max) and PAV_(max). The first is that the heart rate is at or belowthe ADL (728). The second is that a search (730) for conduction isperformed. The third is that a ventricular event is sensed during theSAV_(norm) or PAV_(norm) interval. Conversely, in this embodiment, theonly time the SAV_(norm) or PAV_(norm) values are utilized is when theheart rate is above the ADL, a ventricular pace has occurred in thepresent cycle and a search is not being performed.

In the illustrated embodiment, the SAV and PAV values will not be set to“norm” values if there is a sensed ventricular event. This assumes thatintrinsic conduction, even if occuring in the interval between AV_(norm)and AV_(max), is preferable to right ventricular pacing. In analternative embodiment not separately illustrated, the AV intervals areset to “norm” values whenever the heart rate exceeds the ADL Rate,whether or not the ventricular event was sensed or paced. In such anembodiment, the search (730) is rendered moot and may be eliminated.This may be an option provided to caregivers and/or patients. Forexample, some caregivers or patients may believe that during periods ofexercise (heart rate above ADL), they feel or perform better withventricular pacing. Thus, the clinician may have the option of selectingor disabling this parameter and/or permitting the patient to selectivelyenable or disable this parameter. As an example, a given patient mayroutinely run for exercise. Intrinsic conduction may be preferable atmost times, even when the heart rate exceeds the ADL rate for otherreasons; however, while running the patient may enable the “norm”values. This may be accomplished with a patient activator in telemetriccommunication with the device or by other communication means. Asanother optional feature, the device may revert to the illustratedembodiment after the expiration of some predetermined period of time(e.g., 1 hour, 4 hours, etc.) after patient selection.

FIG. 15 is a flowchart illustrating a process wherein the EMS values aredetermined based upon QT intervals, as described herein. As most of thesteps are the same or similar to those indicated in FIG. 14, only thosethat are different will be described. Equations (705) replace those of(704). Specifically, EMSs and EMSP are functions based upon the QTinterval as described in detail above. The initialization process (800)is similar to that of FIG. 14, but step (814) is eliminated as restingEMS is not measured. As before, the variables are recalculated for eachcardiac cycle. For initialization (800), the process may utilize one oftwo methods. In the first, multiple cardiac cycles pass and in at leastone, ventricular pacing is delivered and in at least one ventricularpacing is withheld. Thus, measured values for all of the variables in(702) and (705) are obtained. In the second method, one or more cardiaccycles are utilized for initialization (800), but ventricular pacing isneither withheld nor “forced.” As such, certain variables might not havemeasured initial values. If that is the case, default values are used ininitialization (800) and actual values are obtained as the processproceeds and those values are utilized.

Steps (726) and (740) are replaced with steps (742) and (744)respectively. In step (742), the QT interval is measured, evaluated orotherwise obtained and provided to the device as a useable parameter. Asventricular pacing has occurred in the cardiac cycle, data for the EMSpis now available to update the EMSp parameter. Conversely, since noventricular event was sensed, the EMSs cannot be updated and thatvariable retains the same value previously set.

In step (744), the QT interval is obtained. Here, a ventricular eventwas sensed so data is available to update the EMSs and that occurs. Inboth steps (742) and (744) the SAV_(max) and PAV_(max) values arerecalculated; however, the EMS value will only change for one of thosecalculations depending upon whether the ventricular event was sensed orpaced.

FIG. 16 is a flowchart illustrating a process wherein the EMS values aredetermined using sensor input. Once again, the steps are generally thesame or similar to those indicated in FIGS. 14 and 15 so that only thosethat are different will be described. As indicated, the EMS formulas(706) are based upon sensor input that is indicative of a parameterassociated with EMS, as described in detail above. For example, thesensor may be a microphone to pick up heart sounds, an impedance sensorto measure impedance values through the ventricle(s) to determinevolume, a pressure sensor, or flow sensor. Steps (746) and (748) replacesteps (742) and (744) respectively. The difference here being thatsensor input is used to measure the appropriate EMS values, as opposedto QT intervals. The remainder of the flowchart includes the same steps.

The present invention has been described in the context of variousembodiments. These embodiments are for illustrative purposes only andare not meant to be limiting, rather the spirit and scope of theinvention may be broader than the specific embodiments provided whichshould not be limiting to the following claims.

1. An implantable medical device (IMD) comprising: a memory storing aresting electro-mechanical systole value (EMSr) and a resting cardiaccycle length (CLr) and a correction factor (P); means for generatingventricular pacing stimuli on expirations of AV intervals; means fordetermining a current cycle length (CL); and processor means coupled tothe memory for setting the value of the AV intervals equal to thecurrent cycle length minus a current EMS value, wherein the current EMSvalue is equal to: EMSr−(CLr−CL)×P).
 2. The IMD of claim 1, wherein P isa value between 0.2 and 0.25.
 3. The IMD of claim 1, wherein P equals0.22.
 4. The IMD of claim 1, wherein the means for determining a currentcycle length comprises means for determining interventricular conductiondelay (IVD) and wherein the processor means utilizes the EMS value asdetermined in claim 1 for cardiac cycles following sensed ventricularevents and uses an EMSp value instead of the EMS value for cardiaccycles following paced ventricular events, wherein EMSp is equal to theEMS value+IVD.
 5. The IMD of claim 4, further comprising means forgenerating atrial pacing stimuli and wherein the means for determining acurrent cycle length further comprises means for responding to sensedand paced atrial events and wherein the means for setting the AVintervals determines a SAV value when atrial events are sensed and a PAVvalue when atrial pacing occurs.
 6. The IMD of claim 5, wherein thememory stores a value for interatrial electromechanical conduction delayresulting from sensed atrial events (IAEMDs), a value for interatrialelectromechanical conduction delay resulting from paced atrial events(IAEMDp) and a value for a P wave sense offset (PSO) and wherein theSAV=CL−EMS+IAEMDs−PSO and the PAV=CL−EMSp+IAEMDp.
 7. The IMD of claim 6,wherein the memory stores an Activities of Daily Living (ADL) rate andfurther comprising: means for determining whether the cycle lengthcorresponds to a heart rate in excess of the Activities of Daily Living(ADL) rate; and means for providing a normalized AV interval that isless than the SAV or PAV when the heart rate corresponding to the CL isgreater than the ADL rate.
 8. The IMD of claim 7, wherein the memorystores a mean normal value for left side mechanical AV intervals (MNV)and wherein means for providing the normalized AV interval provides aSAVnorm value for sensed atrial events, the SAVnorm value equalingIAEMDs−PSO−IVD+MNV, and a PAVnorm value for paced atrial events, thePAVnorm value equaling MNV+IAEMDp−IVD.
 9. The IMD of claim 8, whereinthe MNV is between 65 and 105 ms.
 10. An implantable cardiac pacemakeradapted to provide pacing mode that promotes intrinsic conduction to ahigh degree while providing properly timed ventricular pacing whenrequired to generally prevent cardiac cycles devoid of ventricularactivity while avoiding unnecessary right ventricular pacing,comprising: atrial and ventricular sense amplifiers responsive tosignals associated with atrial and ventricular depolarizations,respectively; atrial and ventricular pacing pulse generators; andcontrol circuitry responsive to the sense amplifiers and triggering thepulse generators, comprising a processor, memory and programming storedtherein, the processor controlled by the programming stored in thememory, the memory storing values for resting cardiac cycle length (CLr)and resting electromechanical systole duration (EMSr) and a correctionfactor (P), the programming comprising instructions defining AVintervals on expiration of which the ventricular pacing pulse generatoris triggered, and further comprising instructions for the processor toperform the following operations: 1) determining a prevailing cardiaccycle length (CL); 2) determining current duration of electromechanicalsystole (EMS) based upon the prevailing cardiac cycle length (CL), theresting electromechanical systole duration (EMSr) and the correctionfactor (P); and 3) defining a subsequent AV interval to facilitateintrinsic AV conduction, while at the same time avoiding overlap of leftatrial contraction and the left ventricular contraction of the precedingcardiac cycle by setting the subsequent defined AV interval based uponthe prevailing cardiac cycle length, the current duration ofelectromechanical systole and the correction value.
 11. The pacemaker ofclaim 10, wherein the stored programming comprises instructions fordetermining interventricular conduction delay (IVD) and wherein theprocessor utilizes the EMS value as determined in claim 10 for cardiaccycles following sensed ventricular depolarizations and uses an EMSpvalue instead of the EMS value for cardiac cycles following generatedventricular pacing pulses, wherein EMSp is equal to the EMS value+IVD.12. The pacemaker of claim 11, wherein the programming further comprisesinstructions for defining AV intervals and comprises instructions forsetting the AV interval to a first duration (SAV) following sensedatrial depolarizations and a second duration (PAV) following generatedatrial pacing pulses.
 13. The pacemaker claim 12, wherein the memorystores a value for interatrial electromechanical conduction delayassociated with sensed atrial events (IAEMDs), a value for interatrialelectromechanical conduction delay associated with generated atrialpacing pulses (IAEMDp) and a value for a P wave sense offset (PSO) andwherein the duration of SAV=CL−EMS+IAEMDs−PSO and the duration ofPAV=CL−EMSp+IAEMDp.
 14. The pacemaker of claim 10, wherein the memorystores an Activities of Daily Living (ADL) rate and wherein theprogramming further comprises instructions for: determining whether theprevailing cardiac cycle length corresponds to a heart rate in excess ofthe Activities of Daily Living (ADL) rate; and defining a normalized AVinterval that is less than the SAV or PAV when the heart ratecorresponding to the prevailing cardiac cycle length is greater than theADL rate.
 15. The pacemaker of claim 14, wherein the memory stores amean normal value for left side mechanical AV delay (MNV) and whereinthe instructions for defining a normalized AV interval compriseinstructions to reduce PAV to a PAVnorm value and SAV to a SAVnorm valuewhere PAVnorm=MNV+IAEMDp−IVD and SAVnorm=MNV+IAEMDs−PSO−IVD.
 16. Thepacemaker of claim 15, wherein the MNV is between 65 and 105 ms.
 17. Thepacemaker of claim 10, wherein P is a value between 0.2 and 0.25. 18.The pacemaker of claim 10, wherein P equals 0.22.